1. Field of the Invention
The present invention relates to a method for manufacturing a display panel used in a display device for displaying text, images, etc.
2. Description of the Related Art
In general, display panels used in electroluminescence (EL) display devices and in liquid crystal display devices are constructed to include display substrates onto which display elements such as, for example, a light emitting element, liquid crystal, and a driving element for driving the light emitting element or liquid crystal are provided. Normally, in order to maintain the function and quality of the display substrate, the display substrate is sealed with a suitable sealing member. Examples of materials used for such a sealing member include metal and glass. The display substrate is affixed to the sealing member by an adhesive and sealed. The quality of sealing of the display substrate in the display panel is an important factor for determining the quality and lifetime of the display device.
FIG. 1 schematically shows how a display substrate is sealed by a sealing member.
As shown in FIGS. 1(a) and 1(b), element layers 32 which will become the display regions are formed through a thin film formation process on one side of a glass substrate 31 which is a type of display substrate 33. FIGS. 1(a) and 1(b) show an example configuration in which a plurality of (twelve) element layers 32 and a plurality of (twelve) display substrates 33 are simultaneously formed on a glass substrate 31 for simultaneously manufacturing a plurality of (twelve) display panels. The position of the glass substrate 31 relative to a sealing glass 34 which is the sealing member placed to oppose the element layers 32 is determined by an image processor or the like which recognizes one or more positioning marks 39 on the sealing glass 34. Then, the glass substrate 31 is moved in the Z direction shown in FIG. 1(a) and is affixed to the sealing glass 34. On the sealing glass 34, an adhesive 35 is applied in advance in a manner to surround the display regions along the shape to be sealed of the display substrates 33 (more specifically, their element layers 32). The surface of the sealing glass 34 opposing the display substrate 33 is engraved through etching or the like to correspond to the shapes and arrangement of the element layers 32. The engraved section 36 of the sealing glass 34 is provided for applying an absorbent or the like for maintaining the characteristics of the display substrate 33 to be sealed. In FIG. 1(b), the glass substrate 31 is not shown.
FIG. 2 schematically shows the cross section of the structure when the glass substrate 31 and the sealing glass 34 are affixed. The glass substrate 31 is held to a supporting member 37 using vacuum suction and affixed to the sealing glass 34 which is placed on a base (not shown). During this process, as shown in FIG. 2, the glass substrate 31 and the sealing glass 34 are pressed towards each other so that a predetermined gap G is formed between the glass substrate 31 and the sealing glass 34. After the gap G is adjusted to the predetermined value, a curing process for the adhesive 35 is applied and the display substrate 33 is sealed by the sealing glass 34. During this sealing process, the width of the portion of the glass substrate 31 and of the sealing glass 34 in contact with the adhesive 35, that is, the seal line width W, is determined by the amount and viscosity of the adhesive 35, the gap G, the magnitude and duration of the applied pressure, etc. Also, a spacer 38 having a cylindrical or a spherical shape with a predetermined diameter, for example, is provided within the adhesive 35 (schematically shown in FIG. 2) so that a predetermined gap G can be obtained using the spacer 38 as a stopper for the pressure application.
Normally, a resinous adhesive is used as the adhesive 35. When a resinous adhesive is used, the material of the resin is selected based on the type of display substrate 33, the object of sealing, etc. However, for some of these resins, the viscosity cannot be adjusted.
For example, for a display substrate used in a display panel of an EL display device, that is, a display substrate 33 onto which an EL element is formed at the element layer 32, because an EL element has characteristics that its heat endurance is low and an EL element is easily degraded by moisture, an ultraviolet curable epoxy resin which has low permeability for water and, in addition, which does not require heating for curing is used as the adhesive 35. Because the ultraviolet curable epoxy resin is not diluted by any solvent, in general, the ultraviolet curable epoxy resin has a high viscosity and cannot be adjusted to a viscosity at which the resin can be easily used. In addition, if the viscosity is adjustedby changing the constitution of the ultraviolet curable epoxy resin, it is difficult to maintain the low water permeability characteristic of the resin.
When a resin having a high viscosity as described above is used as the adhesive 35, it is necessary, during the affixing of the glass substrate 31 to the sealing glass 34 as described above, to apply a higher pressure on the affixing surfaces of the substrate 31 and the sealing glass 34 to allow the gap G to reach a desired value and, at the same time, secure the seal line width W. However, if the magnitude of the applied pressure is rapidly increased, the adhesive 35 having a high viscosity cannot change its shape to respond to the change in the pressure, and affixing defects such as shown by dotted lines in FIG. 3 which is a plan view of the sealing glass 34 may be generated.
More specifically, when the adhesive 35 does not follow the change in the applied pressure and the gap G does not uniformly reach a desired value, defects such as (A) creation of a seal path through which the gas remaining in the inner space to be sealed can escape, (B) instability of the seal line width W, and (C) deviation of the adhesive 35 from the predetermined sealing position, may be generated. These are marked with the respective labels A, B, and C in FIG. 3. These affixing defects not only cause shape defects, but may also adversely affect the quality and lifetime of the EL display device by, for example, creating a sealing defect, causing the pressurized gas to remain inside, or increasing water permeability.
In addition to affecting the above described display substrate onto which an EL element is formed, the above-described disadvantages are common when sealing any display panel, such as, for example, a liquid crystal display substrate and a plasma display substrate, when the display substrate is sealed using a suitable sealing member and a highly viscous resin adhesive.